Silicon carbide reinforced silicon carbide composite

ABSTRACT

This invention relates to a process comprising the steps of: 
     a) providing a fiber preform comprising a non-oxide ceramic fiber with at least one coating, the coating comprising a coating element selected from the group consisting of carbon, nitrogen, aluminum and titanium, and the fiber having a degradation temperature of between 1400° C. and 1450° C., 
     b) impregnating the preform with a slurry comprising silicon carbide particles and between 0.1 wt % and 3 wt % added carbon 
     c) providing a cover mix comprising: 
     i) an alloy comprising a metallic infiltrant and the coating element, and 
     ii) a resin, 
     d) placing the cover mix on at least a portion of the surface of the porous silicon carbide body, 
     e) heating the cover mix to a temperature between 1410° C. and 1450° C. to melt the alloy, and 
     f) infiltrating the fiber preform with the melted alloy for a time period of between 15 minutes and 240 minutes, to produce a ceramic fiber reinforced ceramic composite.

This application is a division of Ser. No. 08/758,715 filed Dec. 2, 1996U.S. Pat. No. 5,840,221.

STATEMENT OF GOVERNMENT SUPPORT

A portion of the subject matter of this invention was developed underthe High Speed Civil Transport/Enabling Propulsion Materials Program,sponsored by NASA through Contract No. NAS3-26385.

BACKGROUND OF THE INVENTION

Reinforced ceramic matrix composites (“CMC's”) are well suited forstructural applications because of their toughness, thermal resistance,high temperature strength and chemical stability. These composites canbe produced by adding whiskers, fibers or platelets to a ceramic matrix.In the fabrication of continuous fiber reinforced-ceramic matrixcomposites (“CFCC's”), the fabrication process usually begins by weavingcontinuous TM fiber tows (e.g., sintered SiC fibers such as Hi-Nicalonor Dow Corning Sylramic™) into a cloth such as 2-dimension 5HS or 8HS,or 3-dimension cloths. The woven fiber cloth is then formed into a panelor shape called a fiber preform. The porosity within the fiber preformis then filled to produce the dense CFCC. The non-brittle nature of theCFCC provides the much needed reliability that is otherwise lacking inmonolithic ceramics.

The enhanced fracture resistance of ceramic matrix composites isachieved through crack deflection, load transfer, and fiber pull-out.Fiber pullout is achieved by having little or no chemical bondingbetween the fibers and matrix, so that the fibers are able to slidealong the matrix. However, it is also known that many fiber-matrixcombinations undergo extensive chemical reaction or interdiffusionbetween the fiber and matrix materials during densification. Suchreaction or interdiffusion can lead to serious degradation in strength,toughness, temperature stability and oxidation resistance. Accordingly,the proper fiber-matrix interface must be selected in order to preventor minimize chemical reactions and interdiffusion.

Surface modification of the fibers is an effective means to controlreaction at the fiber-matrix interface. This can be accomplished bycoating the fibers with a suitable ceramic. Equally important, asuitable ceramic coating also allows the debonding of the fiber's matrixinterface and enables the fiber to pull out from the matrix and slidealong the matrix, thus increasing the fracture toughness of thecomposite. Coated silicon carbide fibers and whiskers are known in. Theart of composite materials. U.S. Pat. No. 4,929,472 (“Sugihara”)discloses SiC whiskers having a surface coated with either acarbonaceous layer or a silicon nitride layer. These surface coatedwhiskers are used as a reinforcing material for ceramics such as SiC,TiC, Si₃N₄, or Al₂O₃. U.S. Pat. No. 4,781,993 to Bhatt discloses a SiCfiber reinforced reaction bonded Si₃N₄ matrix wherein the SiC fibers arecoated with an amorphous carbon layer and an overlayer having a highsilicon/carbon ratio covering the amorphous layer. U.S. Pat. No.4,642,271 to Rice discloses BN coated ceramic fibers embedded in aceramic matrix. The fibers may be SiC, Al₂O₃ or graphite, while thematrix may be SiO₂, SiC, ZrO₂, ZrO₂-TiO₂, cordierite, mullite, or coatedcarbon matrices. U.S. Pat. No. 4,944,904 to Singh et al. discloses acomposite containing boron nitride coated fibrous material. Carbon orSiC fibers are coated with BN and a silicon-wettable material and thenadmixed with an infiltration-promoting material. This mixture is formedinto a preform which is then infiltrated with a molten solution of boronand silicon to produce the composite.

The densification of green CFCC's is more difficult than that of greenmonolithic ceramics. Conventional sintering of a green ceramic matrixreinforced with sintered fibers is not possible, as the green ceramicmatrix has rigid inclusions. Densification of green CFCC's can, however,be achieved by chemical vapor infiltration (“CVI”) or molten siliconinfiltration. Molten silicon infiltration is the preferred methodbecause it is less time consuming and more often produces a fully densebody than the CVI process. For high temperature applications, fulldensification is necessary for good thermal and mechanical propertiesand for preventing rapid oxidation/degradation of the reinforcements orreinforcement coating. For example, desirable characteristics for CFCC'sused in air transport applications include a high thermal conductivity,high tensile strength, high tensile strain and a high cyclic fatiguepeak stress. One conventional CFCC fabricated by state-of-the-artchemical vapor infiltration processing has been found to have a thermalconductivity of only about 4.7 BTU/hr.ft.F at 2200° F., and a cyclicfatigue peak stress of only about 15 ksi (about 105 MPa) using aHi-Nicalon™ fiber. It is believed the low thermal conductivity andcyclic fatigue peak stress of this CVI material is due to the material'srelatively high porosity (typically 10-20%) which is common for CVIprocesses. According, the art has focused upon densification by siliconinfiltration.

Densification by silicon infiltration has been practiced for monolithicceramics, such as reaction-bonded silicon carbide, for many years. Thisprocess, as described in U.S. Pat. No. 3,205,043 to Taylor, involvesinfiltrating molten silicon through the pores of a green body containingalpha silicon carbide and carbon. The silicon reacts with the carbon toform beta-SiC, which then bonds the alpha-SiC grains together. Theportion of the infiltrated molten silicon which does not react with thecarbon solidifies upon cooling, thereby filling the pores of the Sicbonded Sic body. This phenomenon is known as siliconization, and resultsin a fully dense end product containing SiC and residual free silicon.Since silicon infiltration does not involve shrinkage of the green body(as is the case with conventional sintering), the final dense product isnear net shape. The art has used silicon infiltration to densifyfiber-containing ceramic composites as well.

U.S. Pat. No. 5,296,311 (“McMurtry”), the specification of which isincorporated by reference, discloses a silicon infiltrated siliconcarbide composite reinforced with coated silicon carbide fibers.McMurtry discloses a process including the steps of:

a) coating SiC fibers with a coating selected from the group consistingof aluminum nitride, boron nitride and titanium diboride;

b) treating the surface of the coated fibers with a mixture of SiCpowder, water and a non-ionic surfactant;

c) preparing a slurry comprising SiC powder and water;

d) impregnating the coated fibers with the slurry using a vacuumdewatering process to form a cast;

e) drying the cast to form a green body; and

f) silicon infiltrating the green body to form a dense SiC fiberreinforced reaction bonded matrix composite.

McMurtry reports that providing the disclosed coatings on SiC fiberslimited both mechanical and chemical bonding with the matrix, and soimproved the strength and toughness of the composite material. However,CFCC's produced in substantial accordance with McMurtry have been foundto have a four point flexure strength at room temperature of only about1 ksi. Since the tensile strength of a ceramic is typically only about60%-90% of its four point flexure strength, these CFCC's likely have atensile strength of only about 0.6-0.9 ksi. Further assuming an elasticmodulus of about 30 million psi, these CFCC's likely have an ultimatetensile strain of less than 0.003% at room temperature. The reason forthese low values is believed to be the low strength of the fiber used inMcMurtry, as well as the partial reaction of the debonding coating withthe molten silicon. Moreover, simple substitution of higher strength SiCfibers, such as Hi-Nicalon fiber, presents more severe degradationproblems because the these higher strength fibers are considered to bemore susceptible to degradation by molten silicon than the SiC fibersused by McMurtry. In particular, these higher strength fibers typicallydegrade in the temperature range of only about 1410-1500° C. while thesilicon infiltration step in McMurtry is undertaken at a temperature ofabout 1500° C.

In addition, one specific problem encountered with SiC reinforced SiCcomposites fabricated by a silicon infiltration process is that the SiCfiber or coating thereon may react with the molten silicon duringinfiltration, resulting in the degradation of the composite's desirableproperties. For example, it has been found that, due to the highreactivity of molten silicon, the BN debonding coating is also attackedduring the silicon infiltration step, resulting in severe degradation ofthe underlying SiC fiber and hence the CFCC properties. To reduce suchattack, a duplex coating concept in which a second “protective” coatingof CVD-SiC is deposited on top of the BN coating has been studied. See,e.g., U.S. Pat. No. 4,944,904. While the CVD-SiC coating is more stablethan the underlying BN coating in the presence of molten silicon, it hasbeen found that molten silicon still dissolves the CVD SiC coatingconsiderably. As a result, the silicon melt infiltration process has tobe conducted at a relatively low temperature (i.e., close to the meltingpoint of silicon, which is 1410° C.) and for a short time (less than 30minutes). Because of this abbreviated infiltration step, the resultingCFCC microstructures often have incomplete silicon infiltration, highporosity and poor thermo-mechanical properties. A second aspect of theconventional process as typified by U.S. Pat. No. 4,889,686 which limitsthe completeness of silicon infiltration is the use of carbon in theimpregnation slurry. During the slurry impregnation step, the coatedfiber tows or fabrics are impregnated with carbon, which is typicallypresent as at least 10 wt % of the slurry. The infiltrated fiber tows orfabrics are then placed in a vacuum furnace and heated in the presenceof molten silicon. The infiltrated carbon quickly reacts with moltensilicon to form a beta SiC matrix. According to McMurtry, the presenceof carbon in the slurry provides a reactant for forming the matrix SiC,and is believed to improve the wetting behavior of molten silicon and soallows the silicon to penetrate deeper into the fiber tow interior. Thebeneficial effects of the impregnated carbon during silicon infiltrationis widely accepted. For example, the General Electric Toughened Silcomp™process uses a slurry with at least 10 wt % carbon. However, since thereaction between silicon and carbon is a highly exothermic one, the heatgenerated by this reaction can cause severe localized heating of thefiber preform to between 100 and 200 degrees C above the intended moltensilicon temperature. Since the stability of the higher strength Sicfibers and some debonding coatings (such as BN) are very sensitive totemperature, degradation of the coatings and the fibers are frequentlyencountered. One approach for decreasing this degradation is to limitthe time and temperature at which silicon infiltration is performed. Asa result of the relatively low temperatures used in the siliconinfiltration step, siliconization is often incomplete and unreactedcarbon remains. Moreover, it has been observed in conventionalprocessing that the silicon/carbon reaction near the surface regions ofthe green CFCC often blocks the subsequent flow of silicon into thegreen CFCC interior, causing localized porous areas; its volume changecauses cracking in the near net-shape components; and unreacted freecarbon in the composite degrades its high temperature oxidationresistance.

In a third aspect of the conventional process, silicon infiltration iscarried out by placing several large chunks of solid silicon at variouslocations on top of the impregnated green material and heating thesilicon to its melting point. In theory, the infiltration process reliesprimarily on the capillary action of the liquid silicon or the gaseoustransport of silicon vapors to permeate the porous green CFCC preformand to react with the impregnated carbon in the preform to form in-situSiC. Although this process works well for monolithic ceramics, whereininfiltration is usually conducted at relatively high temperatures (atleast 1750° C.) which make the infiltration kinetics very fast, it doesnot work well with fiber preforms. Due to the limited thermal stabilityof the higher strength fibers and interface coating system, thetemperature in CFCC's during molten silicon infiltration has to be keptvery close to the melting point of silicon (about 1410° C.). Since theinfiltration kinetics are very slow at these lower temperatures, ittakes an exceedingly long time for the molten silicon to wick or spreadto areas not directly under a silicon chunk. This results in eithernonuniform infiltration characterized by many porous areas or severefiber/coating attack if the infiltration process is allowed to proceedfor a much longer time to complete the infiltration. In either case, aninferior CFCC is produced. Secondly, with this technique it is alsoextremely difficult to control the net amount of silicon infiltratedinto the fiber preform. As a result, extra silicon in the form ofsurface lumps is usually observed on the CFCC exterior. Althoughpost-infiltration machining of these lumps can be undertaken, it notonly increases the fabrication cost of the CFCC, it also degrades itsCFCC properties.

Therefore, conventional CFCC's made by silicon infiltration processestypically contain fibers which are either heat resistant at typicalsilicon infiltration temperatures but have low strength, or containfibers which have high strength but are susceptible to degradation attypical silicon infiltration temperatures. Conventional CFCC's made byCVI processes typically have high porosity, and so have low thermalconductivity, low cyclic fatigue peak stress at high temperatures, andlow resistance to oxidation.

Accordingly, there is a need for a CFCC having a high thermalconductivity, a high cyclic fatigue peak stress at high temperatures, ahigh ultimate tensile strain, and a high ultimate tensile strength.

In a fourth aspect of the conventional process, it has been observedthat the surface texture of the composite after silicon infiltration hasthe same highly rough woven structure of the fiber preform. Forapplications such as turbine or aerospace components that need anaerodynamic surface finish, such a rough surface can result in reducedperformance. One proposed solution to the surface roughness problem isto deposit a layer of CVD SiC on the impregnated preform surface andthen machine it to the desired surface finish. The disadvantage of thisapproach is that it is difficult and costly to machine the hard CVD SiCcoating.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a preferredprocess comprising the steps of:

a) providing a fiber preform comprising a non-oxide ceramic fiber havingat least one coating, the fiber and coating each optionally having adegradation temperature of between 1410° C. and 1450° C., the coatingcomprising an element selected from the group consisting of carbon,nitrogen, aluminum and titanium,

b) impregnating the preform in a porous mold with a slurry comprisingsilicon carbide particles and between 0.1 and 3 wt % added carbon toproduce an impregnated green body,

c) providing a cover mix comprising:

i) an alloy comprising a metallic infiltrant and the element, and

ii) a resin,

d) placing the cover mix on at least a portion of the surface of theimpregnated green body,

e) heating the cover mix to a temperature between 1400° C. and 1500° C.to melt the alloy (optionally, between 1410° C. and 1450° C.), and

f) infiltrating the green body with the melted alloy for a time periodof between 15 minutes and 240 minutes, to produce a ceramic fiberreinforced ceramic composite.

Also in accordance with the present invention, there is provided asilicon carbide fiber reinforced silicon carbide composite having anultimate tensile strain of at least 0.3% (preferably at least 0.6%) at2200° F., an ultimate tensile strength of at least 20 ksi (preferably atleast 30 ksi) at 2200° F., and having a thermal conductivity of at leastabout 5.5 BTU/hr.ft.F at 2200° F. and at least about 8 BTU/hr.ft.F at22° C., a cyclic fatigue peak stress of at least 20 ksi at 2200° F. for1000 hours, and less than 10 vol % in-situ formed beta silicon carbide.dr

DESCRIPTION OF THE FIGURES

FIG. 1 is a photomicrograph at a 750× magnification of silicon carbidefiber reinforced silicon carbide composite conventionally infiltratedwith molten unalloyed silicon.

FIG. 2 is a photomicrograph at a 750× magnification of silicon carbidefiber reinforced silicon carbide composite of the present inventioninfiltrated with an alloy of silicon presaturated with carbon.

FIG. 3 is a photomicrograph at a 50 × magnification of silicon carbidefiber reinforced silicon carbide composite of the present invention,which shows essentially no porosity in the matrix regions over arelatively large region.

FIG. 4 is a photomicrograph at a 37.5× magnification of a composite inwhich silicon infiltration occurred after impregnation with a SiC slurryhaving a high concentration of added carbon.

FIG. 5 is a graph comparing the cyclic fatigue peak stress of the CFCCof the present invention versus that of a state-of-the-art CFCC made byCVI process.

FIG. 6 is a graph comparing the thermal conductivity of the composite ofthe present invention and a composite densified by chemical vaporinfiltration.

FIG. 7 is a drawing of a stackable fixture-fiber preform assemblypreferably used to make the composite of the present invention.

FIG. 8 is a photograph of a CFCC produced in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

For the purposes of the present invention, the “degradation temperature”of a fiber or coating is the temperature at which the fiber or innercoating begins to degrade after one hour exposure to molten silicon atthat temperature, as observable under an optical microscope at 750×. Anexample of degraded fibers and degraded coatings is provided in FIG. 1.Similarly, “thermal conductivity” is measured by using a laser flashtest to calculate the thermal diffusivity of the material. Similarly,“cyclic fatigue peak stress” is measured by heating the test bars usedin ASTM C1275-94 to 2200° F. under no load, increasing the load to thetest load within about 10-30 seconds, holding at the test load and 2200°F. for two hours, decreasing the test load to zero within about 10seconds, and repeating this cycle for at least 1000 hours. The cyclicfatigue peak stress is the maximum test load which survives this cyclingfor at least 1000 hours. In addition, “impregnation” refers to theaddition of silicon carbide particles to fill the porosity of the fiberpreform, while “infiltration” refers to the addition of a molten metalsuch as silicon to the impregnated fiber preform.

In a first aspect, the fiber preform is soaked in a surfactant solutionhaving no ceramic particles therein. In a second aspect of the presentinvention, the soaked preform is placed in a covered container having aliquid medium (preferably an aqueous SiC slurry), and a vacuum in drawnon the system, thereby eliminating trapped bubbles from the interior ofthe soaked preform. In a third aspect, the carbon source in the SiCimpregnation slurry is limited to an amount of between about 0.1 wt %and 3 wt % added carbon (preferably between 0.1 and 1 wt % added carbon)of the slurry, thereby virtually eliminating the exothermic reaction ofimpregnated carbon with silicon and allowing a longer siliconinfiltration step, which leads to more complete silicon infiltration. Ina fourth aspect, the impregnation slurry further comprises a bimodalblend of silicon carbide, a small amount of boron carbide, and nobinder. In a fifth aspect, the fiber preform is impregnated with theslurry by pressure casting in a porous mold, thereby promoting morecomplete impregnation of the preform. In a sixth aspect, the siliconcarbide slurry impregnation process is allowed to proceed past the pointof complete impregnation of the fiber preform so that green siliconcarbide completely covers, or “overgrows”, the surface of the preform,thereby allowing subsequent finishing to provide a low surfaceroughness. In a seventh aspect of the present invention, the siliconused to infiltrate the impregnated fiber preform is saturated withcarbon, thereby reducing the driving force for the dissolution of theCVD SiC coating in the molten silicon and allowing a longer infiltrationtime and leading to more complete densification. In an eighth aspect,the silicon to be used in the silicon infiltration step is processedinto a cover mix comprising silicon and a resin, and this mix is spreadacross a face of the CFCC fiber preform, thereby providing a more evendistribution of the silicon during infiltration. In a ninth aspect, useof a high strength, but low degradation temperature fiber in conjunctionwith the above modifications allows the silicon infiltration step to beperformed at temperatures (i.e., between about 1410° C. and 1450° C.)and times (i.e., between about 15 and 240 minutes) which do not degradethe high strength fiber but still allow for complete infiltration. In atenth aspect, there is provided a non-oxide ceramic fiber reinforcedceramic composite which has high thermal conductivity, high cyclicfatigue peak stress, high ultimate tensile strength, and high ultimatetensile strain.

In the first aspect, prior to slurry impregnation, the fiber preform issoaked in a surfactant solution having no ceramic particles therein.Whereas the soak solution in the McMurtry process contained SiCparticles, the soak solution of the present process preferably has nosuch particles. Without wishing to be tied to a theory, it is believedthe McMurtry impregnation process (which provided vacuum dewateringthrough a filter-paper lined glass funnel) was prone to providingincomplete impregnation, and so SiC particles were added to the soaksolution as a way of insuring the fibers were at least partially coatedwith SiC particles. It has been found the present process (whichincludes pressure impregnation through a porous mold) provides betterimpregnation of the slurry into the fiber preform than the McMurtryprocess, thereby eliminating the need for including SiC particles in thesoak solution. Typically, the surfactant solution used to soak the fiberpreform comprises deionized water and no more than about 2 wt % of anon-ionic wetting agent, such as 2 wt % Triton X-100 surfactant,comprising iso-octylphenoxypolyethoxyethanol.

In a second aspect of the present invention, the soaked preform isplaced in a covered container having a liquid medium (preferably anaqueous SiC slurry), and a vacuum in drawn on the system, therebyeliminating trapped bubbles from the interior of the soaked preform.

In the third aspect, the amount of the added carbon in the impregnationslurry is limited to amounts below conventional levels. It has beenunexpectedly found that SiC fiber preforms having a BN/SiC duplexovercoat can be completely infiltrated with silicon despite using lowerthan conventional amounts of carbon in the impregnation slurry. Thisfinding is suprising because it was not previously known in the art howa low- carbon slurry (i.e., a slurry having between 0.1 wt % and 3 wt %added carbon) could be successfully used in a silicon infiltrationprocess conducted at relatively low temperatures (i.e, between about1410° C. and 1450° C.). Although Example 1 of McMurtry discloses aslurry having no added carbon, the silicon infiltration temperature ofthat Example was 1500° C. Without wishing to be tied to a theory, it isbelieved that the SiC fiber network in the green body acts as atransmission conduit for the molten alloy in the absence of carbon, andso the lower carbon level does not adversely affect the wetting behaviorof silicon.

Moreover, reduced carbon levels in the impregnation slurry also reducesthe extent of the exothermic reaction in the subsequent siliconinfiltration step between the infiltrating molten silicon and carbon toform in-situ beta silicon carbide. This condition allows a largertemperature processing window, as it is no longer necessary to makeallowance for the anticipated 100° C. to 200° C. temperature overshootarising from the carbon-silicon silicon reaction that may degrade thecoating and fiber. This allows the use of high strength Hi-Nicalon™fibers which are susceptible to degradation during the siliconinfiltration conditions which were typically required for completeinfiltration of the preform.

By reducing the carbon content of the slurry, many of the problemsassociated with traditional melt infiltration processes are eliminated,and a better composite with a fully dense and uniform matrix, littleporosity, no unreacted residual free carbon, no matrix cracking,improved dimensional control, and no coating/fiber degradation isproduced.

The added carbon in the slurry is typically present as particulatecarbon, colloidal carbon, or carbon-yielding resins. The added carbon iscalculated on the basis of the carbon char remaining after pyrolysis ofthe added carbon source.

In preferred embodiments of the present invention, the infiltrationslurry comprises between about 0.1 wt % and 3 wt %, more preferablybetween 0.1 wt % and 1 wt % added carbon.

Therefore, in preferred embodiments, there is provided a processcomprising the sequential steps of:

a) providing a fiber preform comprising silicon carbide,

b) impregnating the preform with a slurry comprising between 0.1 wt %and 3 wt % added carbon, and

c) infiltrating the preform with a matrix alloy comprising silicon.

When this process is followed, the resulting CFCC typically has lessthan 10 volt (preferably less than 3 vol %) insitu formed beta siliconcarbide.

In the fourth aspect, the SiC impregnation slurry also comprises abimodal blend of alpha silicon carbide, a small amount of boron carbide,and no binder component. The silicon carbide component of the slurrytypically comprises a fine component having a particle size of betweenabout 0.1 and 0.8 μm, and a coarse component having a particle size ofbetween about 1 and 15 μm. Preferably, the fine component comprisesbetween 25 wt % and 55 wt % of the slurry, while the coarse componentcomprises between 1 wt % and 30 wt % of the slurry. The bimodal natureof the silicon carbide provides higher packing in the porous preform andtherefore provides lower porosity in both the green body and thedensified CFCC. It has been found that that using a fine unimodal mix ofSiC particles produces poor packing, excessive shrinkage, and excessivedrying, while a coarse unimodal SiC particle mix can not fully penetratethe fiber bundles. Boron carbide is typically present in an amount ofbetween 0.5 wt % and 5 wt % of the slurry. The boron carbide componentprovides the advantage of improving the composite's oxidationresistance. It is believed that, when a crack occurs, the boron oxidizesand heals the crack. It was also unexpectedly found that removing thebinder component from the slurry (which is disclosed in McMurtry assodium silicate) did not decrease the strength of the green body or thedensified CFCC. Typically green bodies require a binder in order to haveacceptable green strength. Accordingly, in preferred embodiments, thereis no binder component in the slurry. The slurry may also includeconventional amounts of defoamers and dispersants In preferredembodiments, the slurry may comprise between 25 and 55 wt % fine siliconcarbide, between 1 and 30 wt % coarse silicon carbide, between 0.5 and 5wt % boron carbide, between 20 and 65 wt % deionized water, between 0and 1 wt % deflocculant, between 0 and 0.2 wt % defoamer, between 0 and0.5 wt % surfactant, and between 0 and 5 wt % carbon source. Preferably,the slurry may have a solids content of between 46 and 75 wt % and a pHof between 7 and 10.5.

In some embodiments, typical formulations of the slurry can include thefollowing components:

Slurry 1 Slurry 2 Slurry 3 Slurry 4 Slurry 5 Total 84.75 83.49 84.2454.51 75.23 SiC  -fine 56.44 50.09 46.33 35.43 48.90  SiC  -coarse 28.3133.40 37.91 19.08 26.33  SiC B₄C 0.59 0.58 1.26 0.34 0.47 DI water 55.137.2 40.1 43.3 59.8 % solids 62 70 69 60 57.5 defloc't 0.3 0.4 0.3 0.2<1% defoamer 0.07 0.05 0.1 0.027 <1% surfac't 0.76 0.73 0.73 0.073 0.1added 4.07 3.99 4.04 1.52 0.8 carbon

In more preferred embodiments, the deflocculant is a copolymer such asSMA 1440H (50% solution) available from ATOCHEM, North America inPhiladelphia, Pa.; the defoamer is DB-31 emulsion, available fromAshland Chemical Co. of Tonawanda, N.Y.; the pH is adjusted with NaOH;the surfactant is a alkyl polyether alcohol such as Triton X-100,available from JT Baker of Phillipsburg, N.J.; and the added carbon isDerusol carbon black dispersion (56% solids), available from Degussa ofFrankfurt, Germany.

In the fifth embodiment, the fiber preform is infiltrated by pressurecasting in a porous mold, thereby promoting more complete impregnationof the preform. In preferred embodiments, both the fiber preform and themold duplicate the geometry and size of the final CFCC component.Typically, the mold is a porous plaster mold, and the cast pressure isbetween about 20 kPa and 200 kPa. Use of the porous mold in conjunctionwith the pressure casting has been found to produce a CFCC having ahigher degree of impregnation than the process disclosed in McMurtry,which included casting under atmospheric pressure through a glass funnellined with filter paper. In addition, whereas the funnel used in theMcMurtry process could provide only a one-way draw, the porous mold ofthe present invention can provide a draw which is uniform throughout thesurface of the preform. The ability to provide a uniform draw throughoutthe surface of the preform allows the impregnation of complex shapes.This ability was not present in the McMurtry process.

Therefore in accordance with the present invention, there is provided apressure casting process for producing an impregnated fiber preform,comprising the steps of:

a) providing a fiber preform comprising:

i) between 20 volt and 80 vol % coated fiber, the fiber comprisingsilicon carbide,

ii) between 20 vol % and 80 vol % porosity,

b) providing a porous mold having a well,

c) placing the fiber preform in the well,

d) contacting the fiber preform with a slurry comprising water andceramic particles to impregnate the porosity of the fiber preform withthe ceramic particles of the slurry, and

e) dewatering the slurry through the porous mold under pressure, to forma green body having a porosity which is lower than that of the fiberpreform.

Preferably, the pressure used during impregnation is between about 20kPa and about 200 kPa. The mold is preferably plaster of paris. Theporosity of the complex shaped-green body produced by this process istypically between 15 vol % and 30 vol %.

In a sixth embodiment, the silicon carbide impregnation process isallowed to proceed past the point of complete impregnation of the fiberpreform so that silicon carbide particles completely cover the surfaceof the preform, thereby allowing subsequent finishing to provide a lowsurface roughness. After demolding and drying the cast, this “overgrown”monolithic SiC layer is retained to provide a much finer surface finish.Furthermore, since the green overgrown surface monolithic Sic layer ismuch softer than the final densified surface but also has good greenstrength, additional surface finishing steps such as green machining canbe easily conducted to give a highly finished surface comparable to thatof the normal monolithic Sic components. This overgrown green body isthen melt infiltrated with the alloy to fill the remaining porousinterstices between the Sic particles and to react with the impregnatedadded carbon to form in-situ Sic both within the fiber preform and onthe monolithic Sic surface layer. The final CFCC will then be convertedto a fully dense composite with a smooth and tailored surface finishthat is difficult or expensive to achieve using other CFCC processessuch as CVI.

The overgrowth process can be conducted on any CFCC shapes includingflat panels and cylinders. In one preferred embodiment, cylindricalfiber preforms are impregnated with a small gap (less than 0.5 cm)between the outer diameter of the cylinder and the mold surface, therebyallowing a monolithic layer to be built up on the outer diameter.Impregnation is continued until an overgrowth layer is also built up onthe inner diameter. After demolding and drying, the inner diameter isscraped to provide a rough surface while the outer diameter is polishedto provide a smooth surface finish. After melt infiltration, a componentwith tailored surface finish (smooth outer diameter and rough innerdiameter) is readily obtained.

Using the overgrowth process, CFCC's having high surface smoothnessexteriors can be obtained economically. This will allow the use oftoughened ceramic composites in many applications where aerodynamicrequirements are also important. In addition, this invention can alsoprovide tailored surface finishes for CFCC's used as combustor linersfor aircraft or gas turbine applications where both heat and gas flowsare key parameters for optimum performance. With this invention, the twosurfaces can be tailored to have a rough surface away from gas flow (foroptimum heat dissipation), and a smoother surface near the gas flow (tooptimize gas flow aerodynamics).

Therefore, in accordance with the present invention, there is provided aprocess for providing a smooth surface on a CFCC, comprising the stepsof:

a) providing a fiber preform comprising:

i) between 20 volt and 80 volt coated fiber, the fiber comprisingsilicon carbide, and

ii) between 20 and 80 volt porosity,

b) impregnating a slurry comprising ceramic particles into the porosityof the fiber preform to form a green body having a lower porosity thanthe fiber preform (preferably, between 15 volt and 30 volt porosity) andan exterior surface, and

c) depositing ceramic particles on the exterior surface of the greenbody to form a monolithic layer of ceramic particles on the exteriorsurface of the green body, and, optionally,

d) machining the monolithic layer to a surface roughness Ra of no morethan 200 microinches (5μm).

In some embodiments, the monolithic layer comprises silicon carbideparticulate and has a porosity of between 30 volt and 60 vol %.

In some embodiments, the process further comprises the step of:

e) infiltrating the green body with a matrix alloy comprising moltensilicon, and

f) finishing the melt-infiltrated composite to a surface roughness Ra ofno more than 50 uinches.

In the seventh aspect of the present invention, carbon is dissolved inthe alloy to be used in the melt infiltration step (preferably to orbeyond the point of saturation), thereby reducing the driving force forthe dissolution of the CVD Sic outer protective coating on the Sic fiberby the molten alloy and allowing more complete alloy infiltration. Witha reduced risk of molten alloy attack upon the CVD silicon carbide outerprotective coating, the alloy infiltration step can be designed for morecomplete densification. Typically, the alloy comprises at least 80 wt %silicon.

Without wishing to be tied to a theory, it is believed the SiC-moltensilicon interaction occurs via a three-step mechanism. First, fine Sicgrains from the CVD outer protective coating dissolve in the moltensilicon as silicon and carbon. After dissolution, the carbonconcentration in the molten silicon immediately adjacent the dissolvedSic coating becomes higher than that of more distant molten regions,thereby producing a carbon concentration gradient in the molten silicon.With this concentration gradient acting as a driving force, the carbonin the carbon-rich region is transported down the concentration gradientto the carbon-poor region. When the moving carbon solute encounters alarge SiC particle in its path, it uses the Sic particle as a nucleationsite and reprecipitates out from the solution and produces larger Sicgrains via recrystallization. The net result of the molten siliconattack is the dissolution of the fine SiC grains from the coating andthe growth of the larger Sic grains elsewhere, so that the Sic coatingis continuously dissolved by the molten silicon even though thesolubility of Sic in silicon is limited. Since the key factorcontrolling the dissolution of the SiC coating appears to be the carbonconcentration gradient, providing a prealloyed silicon having dissolvedcarbon therein can reduce or eliminate the formation of the carbonconcentration gradient in the molten silicon, and the transport processresponsible for allowing continued removal of the dissolved carbon willnot occur. Since silicon carbide fibers are often fine-grained and soare susceptible to the same degradation mechanism discussed above, thesilicon-carbon alloy should also hinder dissolution of fine-grained Sicfibers as well. With these problems minimized or eliminated, the meltinfiltration process can proceed more completely. Moreover, providing acarbon solute in the molten silicon also has the effect of lowering themelting point of the silicon, thus allowing lower temperatures to beused and reducing the risk of degrading the SiC fibers. Accordingly,using a silicon-carbon alloy has the dual benefit of hindering SiCcoating dissolution (and by the same mechanism, fine-grained SiC fiberdissolution) and lowering the required processing temperature.

One embodiment of the alloy of the present invention can be made byadding between about 0.003 wt % and 10 wt % carbon to molten silicon. Itis typically made by simply mixing silicon and carbon powders andmelting them at a temperature higher than the melt infiltrationtemperature. The alloy is then typically cooled to a solid, and thesolid is crushed into usable size particles.

It is also believed that dissolving nitrogen into molten silicon can beeffective in reducing silicon attack on coatings comprising nitrogen,such as boron nitride coatings. Therefore, in accordance with thepresent invention, there is provided a process comprising the steps of:

a) providing a fiber preform comprising a non-oxide ceramic fiber havingat least one coating, the coating comprising an element selected fromthe group consisting of carbon, nitrogen, aluminum and titanium,

b) heating a matrix alloy comprising a metallic infiltrant (preferablysilicon) and a predetermined amount of the element dissolved therein,and

c) infiltrating the fiber preform with the matrix alloy.

Also in accordance with the present invention, there is provided acomposite comprising:

a) a fiber preform comprising a non-oxide ceramic fiber having at leastone coating, the coating comprising an element selected from the groupconsisting of carbon, nitrogen, aluminum and titanium, and

b) a matrix alloy, wherein the matrix alloy comprises the elementdissolved therein.

In preferred embodiments of this composite, the non-oxide fiber iscoated by an inner debonding coating of boron nitride and an outerprotective coating of CVD silicon carbide, and the matrix alloycomprises boron and carbon dissolved therein.

Typically, the metallic infiltrant of the matrix alloy is silicon.However, other metallic infiltrants which melt at temperatures lowerthan the degradation temperature of the non-oxide fiber selected for thefiber preform and which are resistant to oxidation can be used. Forexample, suitable metallic infiltrants include silicon, aluminum and anyother metal having a melting point lower than the degradationtemperature of the fiber, and mixtures thereof. When silicon is selectedas the metallic infiltrant, it generally comprises at least 80 wt %,more preferably at least 95 wt %, of the matrix alloy. In some preferredembodiments suitable for use with duplex coatings of an inner debondingcoating of boron nitride and an outer protective coating of siliconcarbide, the alloy comprises:

a) between 80 wt % and 99.997 wt % silicon, b) between 0.003 wt % and 10wt % carbon, and c) between 1 wt % and 10 wt % boron.

In embodiments wherein at least one coating comprises carbon, such assilicon carbide, the alloy comprises at least 90 wt % silicon and atleast about 0.003 wt % dissolved carbon as the element. In embodimentswherein at least one coating comprises nitrogen, such as boron nitrideor aluminum nitride, the alloy can comprise at least 1 wt % nitrogen asthe element. In some embodiments wherein at least one coating comprisesaluminum, such as aluminum nitride, the alloy comprises at least 1 wt %dissolved aluminum. In some embodiments wherein at least one coatingcomprises titanium, such as titanium diboride, the alloy comprises atleast 1 wt % dissolved titanium. In practice only very small amounts ofthe element need be added to the alloy so that the element saturates thealloy. In some embodiments, the carbon is present in an amountcorresponding to at least 50% of its saturation level in the alloy whenthe alloy is heated to 1410° C.

In the eighth embodiment, and in order to facilitate the infiltration ofthe alloy, there is provided a cover mix comprising silicon and a resin.The cover mix is placed on at least one face of the green CFCC preformprior to the infiltration step for more even distribution of the siliconduring infiltration. In one embodiment of the cover mix suitable for usewith simple CFCC preform shapes (such as a flat panel), an amount of thecover mix containing substantially the same amount of silicon needed tofully densify the preform is made into a flat bed having the same lengthand width as the preform. The fiber preform is then placed directly ontop of the cover mix bed and the combination is placed in the furnace.Since every portion of one surface of the fiber preform is in directcontact with the cover mix, the maximum distance needed to be traversedby the silicon in order to fully infiltrate the green CFCC is greatlydecreased (usually to no more than 0.3 cm), and full and uniform meltinfiltration is obtained. When melt infiltration is completed, theremnant of the cover mix is a porous SiC sponge that separates easilyfrom the densified CFCC part.

Also, because both the area to be infiltrated and the total amount ofsilicon provided can be precisely controlled, there is very littleexcess silicon, as-processed surfaces appear very clean, and noadditional machining is needed.

Another embodiment of the cover mix is more suitable for use withcomplex-shaped preforms. This cover mix comprising silicon and resin isfirst formed into a green thin shape duplicating the surface contour ofthe preform (typically, by a traditional ceramic powder formingtechnique such as pressing with a properly designed fixture). The mix isthen placed in an oven to cure the resin, thereby forming a freestanding cover blanket having the same contour of at least one face ofthe complex shaped preform and with the desired amount of siliconrequired for infiltration. The cured cover blanket (as a monolith or insegments) is then fitted on top of the fiber preform to provide anintimate contacting and uniform silicon infiltration source. Therefore,in accordance with the present invention, there is provided a processfor uniformly infiltrating a porous body with an infiltrant, the porousbody having a surface, comprising the steps of:

a) providing a cover mix comprising an infiltrant material and a resin,the mix having a form adapted to intimately contact at least a portionof the porous body,

b) placing the cover mix on at least a major portion of the portion ofthe surface of the porous body to be infiltrated,

c) heating the cover mix to a temperature sufficient to melt theinfiltrant material and infiltrate the pores of the porous body with themolten infiltrant.

In preferred embodiments, the cover mix comprises between 80 w/o and 98w/o alloy and between 2 wt % and 15 wt % resin, and more preferablyfurther comprises between 1 wt % and 5 wt % added carbon. In preferredembodiments used with fiber preforms, the matrix alloy comprises siliconpresaturated with at least one element of the fiber coatings, asdescribed above. When silicon is selected as the metallic infiltrantcomponent of the alloy, at least 50 wt % of the silicon is typicallypresent in grain sizes of no more than 4 mm. In some embodiments, theresin comprises a liquid phenolic resin.

In especially preferred embodiments, there is provided a process forsiliconizing a porous silicon carbide body having a surface, comprisingthe steps of:

a) providing a cover mix comprising silicon and a resin,

b) placing the cover mix on at least a portion of the surface of theporous silicon carbide body, and

c) heating the cover mix to a temperature sufficient to melt the siliconand infiltrate the pores of the porous silicon carbide body with themelted silicon.

In other preferred embodiments, the infiltrant material comprisessilicon and the amount of silicon in the cover mix constitutes a volumewhich is between 100% and 200% of the volume of porosity of the porousbody. In others, the surface of the porous body has a contour and thecover mix is shaped to correspond to the contour of the surface of theporous body. In others, the cover mix is placed on the face of theporous body in a way such that the longest distance between any portionof the porous body and the cover mix is no more than 1 cm. In others,the face of the porous body has a curved contour, the resin of the covermix is cured, and a portion of the cover mix has a shape substantiallysimilar to the contour of the face of the porous body.

In the ninth embodiment, use of at least some of the above modificationsallows the silicon infiltration step to be performed at a relatively lowtemperature (i.e., between about 1410 and 1450° C.) for a short timeperiod (i.e., about 20 to 60 minutes) which prevents degradation of thefiber but still allows for complete infiltration.

Therefore, in accordance with the present invention, there is provided aprocess comprising the steps of:

a) providing a fiber preform comprising a non-oxide ceramic fiber havingat least one coating, the coating comprising a coating element selectedfrom the group consisting of carbon, nitrogen, aluminum and titanium, atleast of the fiber and the coating having a degradation temperature ofbetween 1410° C. and 1450° C.,

b) impregnating the preform with a slurry comprising silicon carbideparticles and between 0.1 and 3 w/o added carbon,

c) providing a cover mix comprising:

i) an alloy comprising a metallic infiltrant and the coating element,and

ii) a resin,

d) placing the cover mix on at least a portion of the surface of theporous silicon carbide body,

e) heating the cover mix to a temperature between 1410° C. and 1500° C.(preferably between 1410° C. and 1450° C.) to melt the alloy, and

f) infiltrating the fiber preform with the matrix alloy for a timeperiod of between 15 minutes and 240 minutes, to produce a ceramic fiberreinforced ceramic composite.

In the tenth embodiment, there is provided a silicon carbide fiberreinforced ceramic composite whose high strength SiC fiber is notdegraded by the melt infiltration step (thereby producing high ultimatetensile strength and strain), and whose porosity is essentially filledby the melt infiltration step (thereby producing high cyclic fatigue andhigh thermal conductivity). In preferred embodiments, there is provideda silicon carbide fiber reinforced silicon carbide composite having anultimate tensile strain of at least 0.3% (preferably at least 0.6%) at2200° F. (using ASTM C1275-94); an ultimate tensile strength of at least20 ksi (preferably at least 30 ksi) (using ASTM C1275-94); a thermalconductivity of at least about 5.5 BTU/hr°ft°F at 2200° F. and at leastabout 8 BTU/hr ft F at room temperature; and a cyclic fatigue peakstress of at least 20 ksi at 2200° F. See FIGS. 5 and 6. It also has aapparent porosity of less than 1%. The composite also typically has lessthan 10 volt in-situ formed beta silicon carbide, preferably less than 3volt.

Suitable fibers for use in the present invention include any non-oxideceramic fiber having a degradation temperature of at least about 1400°C., preferably at least 1410° C. Some suitable fibers include non-oxideceramic fibers such as carbon and silicon carbide fibers. In oneembodiment, sintered silicon carbide fibers are used. In otherembodiments, fibers comprising silicon carbide manufactured by NipponCarbon Company, under the name of Hi-Nicalon™, or SiC fibersmanufactured by Dow Corning, under the name of Sylramic™, are used. Somefibers which comprise silicon carbide, such as the Hi-Nicalon™ material,have the characteristic of high strength (i.e., a strength of at least200 MPa, and preferably at least 300 MPa) but degrade at relatively lowtemperatures (i.e., these fibers degrade when exposed to a moltensilicon at temperatures of between 1410° C. and 1450° C., and in somecases between 1410° C. and 1420° C., for one hour). When such highstrength, moderate temperature fibers are used, the above-describedaspects of the present invention directed towards reducing the severityof the melt infiltration step are advantageously used.

If a coating is used upon the fibers, it is preferable to use anon-oxide ceramic coating, such as AlN, BN or TiB₂. If a BN coating isused, it preferably has a thickness of between about 0.1 to 3 μm, morepreferably between about 0.3 to 2 μm, and is usually used as an innerdebonding coating. If an AlN coating in used, it preferably has athickness of between about 1-15 μm. If a silicon carbide coating isused, in particular as the outer protective layer of a duplex coating,then its preferred thickness is between 1 μm and 5 μm. This coating isalso susceptible to molten silicon at high temperatures, so theprocesses of the present invention help this coating survive theinfiltration step as well.

In one preferred process for making the invention, the slurry comprisesabout 1 wt % to 30 wt % coarse silicon carbide, about 25 wt % to 55 wt %fine silicon carbide, no binder, between 0.5 wt % and 5 wt % boroncarbide, and between 21 wt % and 26 wt % deionized water. The slurry ismilled for between 1 and 4 hours in order to insure its homogeneity. ThepH of the slurry is adjusted to between 8 and 10 by adding ammoniumhydroxide to the slurry. After milling, the slurry is diluted with 34-38wt % deionized water to produce a slurry having a silicon carbide solidscontent of 57-58 wt %. A carbon source is added to the slurry so thatfrom 0.1 wt % to 1 wt % added carbon is present in the slurry.Concurrently, an appropriate amount of sintered SiC fiber in the form ofa woven preform is soaked in a solution of water containing about 2% orless of a non-ionic wetting agent, such as Triton X-100 surfactant. Thepreform is then immersed in an aqueous silicon carbide slurry and avacuum is drawn in order to purge bubbles from the preform. Thesurfactant-treated fiber preform is then laid in the porous plastermold. The slurry is then poured into the porous mold. Pressure (20-200kPa) is then applied to the slurry to promote Sic particle impregnationof the preform and dewatering. The excess slurry is removed from thegreen part, and the resulting cast is then allowed to fully dry to formthe green body. The green body is then completely densified by siliconmelt infiltration. The temperature range for silicon infiltration isbetween 1400° C. and 1500° C. In some embodiments usingtemperature-sensitive fibers, the melt infiltration is carried out atbetween 1410° C. and 1450° C., more typically between 1400° C. and 1420°C. Under these conditions, the duration of the infiltration can bebetween about 15 minutes and 4 hours, preferably for between about 20minutes and about 40 minutes. The process is preferably carried outunder vacuum (in order to eliminate gas bubbles in the densified body),but can be carried out in inert gas under atmospheric pressure.

Typically, the composite comprises between about 20 volt to 80 voltcoated fiber (more typically between about 40 volt and 70 vol %);between about 1 volt and 79 volt infiltrated silicon carbide (moretypically between about 15 volt and 30 vol %), and between about 1 voltand 79 volt infiltrated alloy (more typically between about 15 volt and30 vol %). The densified matrix portion of the CFCC typically comprisesless than 1 volt apparent porosity.

If the silicon carbide feed material has significant contamination (forexample, has at least 50 angstrom thick layer of silica), then the alloyinfiltration step can be preceded by a silica reduction step, whereinthe green body is subjected to temperatures of between about 1300° C. to1350° C. for about a half hour in a reducing atmosphere. Since many meltinfiltration furnaces have graphite heating elements, the reduction canbe designed to occur in the same melt infiltration furnacing run justprior to actual infiltration of the alloy, and as part of thetemperature ramp up cycle.

EXAMPLE I

This example examines the effect of adding small amounts of carbon tothe molten silicon to produce a carbon saturated silicon alloy for themelt infiltration step.

About 94 gms of silicon powder (30-80 mesh in size) was mixed with about1 gm of Raven 1255 carbon black and about 5 gm of SB 95 elemental boron.The mixture was loaded into a graphite crucible coated with a BN powderslurry (to keep the alloy from sticking to the graphite crucible). Thecrucible was then placed in a vacuum furnace and heated under vacuum toabout 1450° C. for 1 hr to completely melt the Si-C-B mixture and forman alloy. After cooling down, the carbon saturated Si alloy was thencrushed to a powder (−16 mesh in size) for use in the preparation of themelt infiltration “cover mix”.

About 91.2 grams of the crushed carbon saturated alloy described abovewas mixed with 6.8 gm of Varcum 29353 liquid phenolic resin and 2 gm ofRaven 1255 carbon black to prepare a melt infiltration cover mix. A SiCfiber preform (Hi-Nicalon™ fiber, 8 harness satin weave), coated with0.5 μm BN and 4 um SiC) was placed in a plaster mold and slurry castwith an aqueous SiC slurry into a green panel. Before the meltinfiltration step, the green panel was first cut into two sections. Oneof the sections was placed on top of a silicon infiltration “cover mix”made from the carbon-saturated silicon, while the other was placed ontop of a silicon infiltration cover mix made from silicon which was notpre-saturated with carbon. The two samples were loaded together into avacuum furnace for melt infiltration. The melt infiltration conditionsused were identical for both samples at 1450° C. for 60 minutes.

After melt infiltration, the samples were cut, cross-sectioned, mountedand metallographically polished for detailed characterization. Opticalmicroscopic examination of the cross sections revealed drasticallydifferent results on the two samples. For the sample that wasmelt-infiltrated with regular silicon without carbon pre-saturation,extensive attack on the SiC, BN coating and the SiC fibers wasencountered (the reaction zones thereof are depicted by light coloredareas of fibers and broken down coatings in FIG. 1). With the debondingcoating and fibers partially destroyed, severe degradation ofthermo-mechanical properties would occur. In fact, the ultimate tensilestrength and strain of this CFCC was found to be only 38.3 ksi and0.38%, respectively. On the other hand, for the sample that wasmelt-infiltrated with pre-alloyed silicon saturated with carbon, therewas no reaction at all (see FIG. 2), and hence, excellent compositeproperties could be obtained. In fact, the ultimate tensile strength andstrain of this CFCC of the present invention was found to be only 54.5ksi and 0.62%, respectively.

FIG. 2 of this example may also be used to examine the effect of usinglower-than-conventional amounts of carbon in the infiltration slurry.FIG. 3 presents a low magnification photomicrograph of the presentinvention which shows how the above-described process providesessentially complete infiltration of the green body and essentially zeroapparent, or “open”, porosity. FIG. 3 can be contrasted with FIG. 4which contains a photomicrograph of a CFCC made by conventionalprocessing using a slurry with at least about 10 wt % added carbon. Incontrast to the complete densification shown in FIG. 3, the lesscomplete processing of the FIG. 4 material results in higher porosity inthe CFCC microstructure and a cracked matrix.

EXAMPLE II

This example discloses a method of preparing and using the cover mix ofthe present invention to infiltrate a complex shaped fiber preform.

A porous Sic fiber preform having a tubular shape (about 18 cm diameterand about 28 cm in height), one domed end and four tabs was prepared andimpregnated with a silicon carbide slurry as previously described.

Concurrently, 94 wt % silicon granules (30-80 mesh commercial grade)with 5 wt % boron (SB 95) and 1 wt % carbon (Raven 1255) were handblended with a spatula. This mixture was then placed in a BN-coatedgraphite box. The BN coating prevents reaction between the alloy and thegraphite as well as sticking. The mixture was heated to 1450° C. forabout 1 hour in vacuum so as to melt the silicon and form a siliconinfiltrant alloy. After cooling, the solid infiltrant alloy was crushedinto particles about −16 mesh in size.

A cover mix containing about 91.2 wt % of the −16 mesh siliconinfiltrant alloy, about 2.0 wt % carbon (Raven 1255), and about 6.8 wt %phenolic resin (Varcum 29353) was prepared by hand blending.

In order to provide intimate contact between the preform and the covermix and to insure the even application of the cover mix upon thepreform, a segmented and stackable graphite internal fixture wasconstructed, as shown in FIG. 7. The fixture 1 is design to form a gap 2between itself and the preform 3 so that the cover mix can be easilypoured into the gap and tamped. For the present example, the gap was setat about 0.635 cm and the amount of the cover mix was selected to beabout 1.4 times that theoretically required to precisely fill the greenpreform.

The fixture 1 comprises separate stackable rings 91, 92 and 93 whichallow tamping of the cover mix at regular intervals. With only ring 91set in place, the cover mix was poured into the gap formed between thefiber preform and ring 91 and tamped. Next, ring 92 was positioned abovering 91 and additional cover mix was poured and tamped. This procedurewas repeated with each higher ring until the green preform innerdiameter was completely contacted by cover mix. A mound of cover mix wasalso placed over the tabs 4 in contact with both the tab surfaces andthe preform.

The green preform/cover mix/fixture assembly was heated to about 120° C.to cure the resin component of the cover mix. Next, the fixture wasremoved one ring segment at a time. The green preform/cover blanketcombination was then heated to a temperature of between about 1410° C.and about 1450° C. in vacuum to melt the alloy component of the covermix and infiltrate the preform with the melted alloy. When meltinfiltration was completed, the remnant of the cover blanket was foundto be a porous SiC sponge which easily separated from the siliconizedcomposite. FIG. 8 shows the infiltrated composite. Since the totalamount of silicon provided was precisely controlled, there was verylittle excess silicon and as-processed surfaces appeared very clean.

EXAMPLE III

This Example examines the effect of overgrowing the infiltration layeron the surface of the fiber preform.

A flat rectangular fiber preform (6 inch×3 inch×0.008 inch) fabricatedfrom a 5 HS Hi-Nicalon SiC fiber weave was slurry cast in a plastermold. During the casting, care was taken to ensure that no build-up ofany additional surface layer of monolithic SiC had occurred. Afterdemolding and drying, the green composite panel was then meltinfiltrated to form a dense composite. The surface texture of this panel(both back and front) was observed to have substantially duplicated thesame roughness of the starting woven fabric. Quantitative surfacemeasurement was conducted on the as-processed CFCC surface using astylus profilometer. A high surface roughness value of 560 μinch wasobtained.

A second similar fiber preform was then fabricated. This time, however,the casting process was allowed to continue after the full impregnationof the preform interior. As a result, an exterior layer of monolithicSiC (at least about 0.010″ in thickness) was deposited on one surface ofthe fiber panel. After demolding and drying, the “green” monolithic SiCsurface was slightly polished with abrasive paper and a hand-heldrubber-bonded diamond wheel to about 10 mil thickness. The “green” panelwas melt infiltrated in the same manner as the first one. The “overcast”surface finish of this panel after melt infiltration was much smootherthan the reverse surface. There was no resemblance to the original roughpreform surface texture. Profilometer surface roughness measurement onthis panel yielded a much smaller number of 85 μinch. It is clear thateven better surface finish can readily be obtained with improved surfacetreatment (such as brushing) or green machining procedure.

It is apparent that the inventive methods and materials described aboveare a substantial advancement in the field of the manufacture of ceramiccomposites. The foregoing descriptions and examples are meant to beillustrative of the various inventive techniques and materials and arenot intended to limit the scope of the invention, which includes allmodifications and variations that fall within the scope of the followingclaims and their equivalent embodiments. For example, the ceramic fiberpreform may include a carbon fiber preform on which the impregnation andinfiltration steps are practiced.

We claim:
 1. A composite comprising: a) a fiber preform comprising anon-oxide ceramic fiber having at least one coating, the coatingcomprising carbon, and b) a matrix alloy, wherein the matrix alloycomprises carbon dissolved therein and no more than 3 volume percentbeta silicon carbide.
 2. The composite of claim 1 wherein the fiberpreform comprises a) fiber comprising silicon carbide, b) an innerdebonding coating of boron nitride coated thereon, and c) an outerprotective coating of silicon carbide, and the matrix alloy comprises:a) at least 80 wt % silicon, and b) between 0.003 wt % and 10 wt %dissolved carbon.
 3. The composite of claim 2 wherein the matrix alloycomprises: a) between 80 wt % and 99.997 wt % silicon, and b) between0.003 wt % and 10 wt % carbon, c) between 1 wt % and 10 wt % boron. 4.The composite of claim 2 wherein the matrix alloy comprises: a) between90 wt % and 99.997 wt % silicon, and b) between 0.003 wt % and 3 wt %carbon.
 5. The composite of claim 2 wherein the matrix alloy consistsessentially of: a) between 80 wt % and 99.997 wt % silicon, b) between0.003 wt % and 10 wt % carbon, and c) between 1 wt % and 10 wt % boron.